It had long been conjectured that today's gallium nitride (GaN) violet laser diodes (VLDs) were a compromise. Their very existence is contingent upon hetero-epitaxial crystal growth, which uses multiple semiconducting layers deposited on top of a dissimilar material such as sapphire (Nichia; Japan) or silicon carbide (Cree; Durham, NC). Researchers believed that homo-epitaxial growth could lead to improved results, including the production of higher-power lasers at ultraviolet wavelengths.
Unipress (Warsaw, Poland) has now proved the concept by growing GaN lasers on GaN crystal substrates. The VLD is one result from research conducted under the "Development of Blue Optoelectronics" project, financed by the Polish government. "We fabricated the VLD using a high-pressure, high-temperature synthesis method," says project leader Sylwester Porowski. "The VLD is a separated-confinement heterostructure having indium gallium nitride (InGaN) quantum-well (QW) active layers."
The success of Unipress came just two years after its first epitaxy system installation. "We have a homemade MOVPE (metal-organic vapor-phase epitaxy) system with T-shape and vertical reactors. It was constructed three years ago, but in fact the first year was devoted to 'improving' the construction," Porowski says. The processing lab was ready in November 2001, and the group fabricated the laser the next month.
The Unipress laser consists of five InGaN QWs sandwiched between n- and p-type GaN layers. The device is completed by a top contacts and copper heatsink. Most importantly, the mirrors are formed by cleaving. "AFM shows atomically flat surfaces, which is one of the advantages of using bulk GaN," says Mike Leszczynski, head of the Unipress Nitride Epitaxy Lab. "We make a back contact as the substrates are highly conductive, which certainly makes life easier. We activate p-type layers by annealing in nitrogen in the MOVPE reactor. Then, after depositing electrical contacts (nickel, gold), we activate them by annealing in air." GaN-on-GaN advantages
Compared to hetero-epitaxy, homo-epitaxy offers a number of advantages for VLDs. These include a reduction in dislocation density by as much as six orders of magnitude together with the reduction of nonradiative recombination. Due to smaller contact metal diffusion through fewer dislocations, it also reduces degradation caused by high currents.
GaN offers better thermal conductivity than sapphire, though not better than silicon carbide (SiC). The conductive GaN substrate also allows the group to design improved current flow geometry compared to devices on insulating sapphire substrates. For laser structures in particular, GaN offers smaller scattering losses, and it is easier to fabricate laser mirrors because it is easier to cleave single crystal substrates. The process costs are more competitive thanks to the elimination of the sacrificial substrate process used by Nichia and others.
There are drawbacks to the process. For example, only smaller diameter crystals are available. This is less serious for the small die characteristic of VLDs. Work is underway to improve the weight and size of GaN crystals by further refinement of the Unipress technology. The relatively high cost of bulk GaN should, like SiC and others, also fall when it reaches mass production. The Unipress approach is also attractive to researchers because GaN-on-GaN provides the highest-quality strain-free and dislocation-free material. market beckons
Unipress has just spun off a small high-tech company called TopGaN. "This will enable us to transfer our technology such as growth of GaN single crystals, MOVPE, molecular beam epitaxy (MBE), and laser processing into commercial products. We see our strongest advantage coming from the high-pressure technology used for growing GaN bulk crystals. At present these are 100 mm2 in size with a dislocation density below 100 cm-2," says Porowski.
TopGaN is focusing on blue lasers as its final products, but before this technology is ready, the company will offer epitaxial structures on bulk GaN crystals and service with high-pressure annealing (up to 1600º C at 10 kbar).
Unipress continues to improve the devices, for example, by reducing threshold current from 30 kA/cm2 to 14 kA/cm2 just before year end by modifying the structure. "What our aim is, of course, is continuous-wave operation," says Porowski. "We are convinced that we will be able to achieve that by more professional mounting and better heat dissipation."